Currently, most electric power-generating solar cells have silicon as as a light-absorbing, semiconducting material and are fabricated using relatively expensive production processes. To make solar cells economically viable, thin-film solar cells have been developed, which are based on thin-film, light-absorbing semiconductor materials, such as copper-indium-gallium-sulfo-di-selenide, Cu(In,Ga)(S,Se)2, also termed CIGS. CIGS-based solar cells have demonstrated power conversion efficiencies exceeding 19%.
Despite the demonstrated potential of CIGS in thin-film solar cells, the toxicity and low abundance of indium and selenium are major impediments to the widespread use and acceptance of CIGS in commercial devices. An attractive alternative for absorber layers of thin-film solar cells is quaternary chalcogenides, particularly copper-zinc-tin-sulfide, Cu2ZnSnS4 (CZTS). CZTS has a direct bandgap of about 1.5 eV and an absorption coefficient greater than 104 cm−1. In addition, CZTS does not include any toxic or low abundance elements.
Thin films of CZTS have been prepared via sputtering of Cu, SnS, and ZnS precursors; hybrid sputtering; pulsed laser deposition; spray pyrolysis of halides and thiourea complexes; and thermal sulfurization of electrodeposited, e-beam deposited, or sol-gel deposited Cu—Zn—Sn precursors.
Bulk quantities of CZTS have been prepared in evacuated quartz ampoules at temperatures between 400° C. to 1150° C. Bulk quantities have also been prepared in a furnace with a source of sulfur, such as H2S.
However, processes for providing CZTS and its chalcogenide analogs in wafer form have not been disclosed.
Hence, there is a need for a safe, robust process to produce CZTS and its chalcogenide analogs in wafer form for non-vacuum based thin-film solar cells.